So my friend's boyfriend wants to give her something custom, but we don't have her exact size. I know she is a 36D, she has a tummy and not much of a butt, so more apple/top heavy. I am small(32A) and pearshaped, so I have trouble translating. He wants it to be a surprise, otherwise I'd just measure her.

It will be hard to estimate exact measurements, but you could make something that is designed to fit a bit of a range (with knit material so it stretches).

Some thoughts for you to think about in estimating. Do you know how accurate her bra size is (has she been fitted or is that just what she buys?) because if it is accurately fitted you can use that information along with bra sizing information to get her bust measurement (there are online calculators that you can estimate with, but from what I can tell her bust would be approximately 36 inches). Other things to think about while estimating. you said she has a tummy, is it larger or smaller than her bust. Consider the same for hips (larger or smaller than tummy/waist). If you think about all these things it may be possible to estimate, but I know I would be nervous about trying.

Well, if her bra fits her, then you know your underbust measurement is 32 and her bust measurement is 40. So from there you have two options: either sneakily obtain other measurements (by finding some excuse to measure existing clothing) or make her something loose-fitting (like a circle skirt with a drawstring waist or a cape).